Field of the Invention
The present invention relates to a sheet processing apparatus, a method of controlling a sheet processing apparatus, and a storage medium.
Description of the Related Art
In the printing market, a print on demand (POD) using digital printing apparatuses has been becoming popular with recent increasing processing speed and image quality in electrophotographic printing apparatuses and ink-jet printing apparatuses. The POD printing has appeared in place of printing using large-scale printing apparatuses and printing methods so as to handle jobs of relatively small lots in short delivery time periods without using the large-scale apparatuses and systems.
Different from the conventional printing methods in which printing plates are made for printing, in the POD printing, in every printing for each print job, original data is edited and processed, print image data is generated, print appearance is set, and then, print processing is performed in the digital printing apparatuses. To add value to the print products, post-processing on the printed sheets may be performed by a sheet processing apparatus. This series of processing to such a print job is defined as a group of the processes associated with the print job, and called a print work flow in the POD.
In the POD print work flow, typically, a front page, insertion pages, and body pages are printed by a plurality of printing apparatuses, and bookbinding processing of the printed sheets is performed by an off-line sheet processing apparatus. By the off-line sheet processing apparatus, saddle stitch processing is performed to a bundle of sheets, and so-called center folding or center fold binding is performed to fold the saddle-stitch processed sheet bundle. Japanese Patent Application Laid-Open No. 2007-50691 discusses a technique for reducing the workload of operators in such a POD print environment by performing shifts, dividing sheet insertion, and discharge destination change to another stacker every time the number of sheet bundles on a stacker reaches a predetermined number of sheets or a predetermined height.
Meanwhile, in the saddle stitching bookbinding, when a bundle of sheets is folded together, the amounts of elongation of the sheets in the folded portions on the outside of the sheet bundle are larger than those on the inside of the sheets. Consequently, portions of formed images in the folded portions of the sheets of the outside are elongated, and damage such as toner peeling and crack may occur at the portions of the formed images in the folded portions.
To solve such a problem, creasing apparatuses called creasers have been known. The creaser puts a fold (crease) in a folding portion of each sheet in advance prior to the execution of folding processing, for example, folding processing of a bundle of sheets in two, so that the sheets of the outside can be easily folded to prevent toner crack. In such known creasing apparatuses, a creasing blade comes in contact with a sheet to put a fold in the contact portion.
In the above-described POD print work flow, there is a use case in which creasing processing is performed by the creaser after printing is performed on a sheet to be the front page by the printing apparatus, to increase the quality of the saddle stitch bookbinding by the off-line sheet processing apparatus.
On the sheet creased by the crease processing, uneven portions are caused. When the uneven sheets are stacked one after another, the weight of the sheets stacked at the upper part is imposed on the uneven portions of the sheets stacked at the lower part, and thereby the crease portions may be crashed.
For example, in a stacking apparatus, a large amount of printed sheets, e.g., 3000 printed sheets, can be stored. In such an environment, when several thousand sheets are stacked therein, a considerable load is applied to the sheets stacked at a lower part, and the crease portions can be crashed easily.
If the creased portions are crashed, it is difficult to obtain full back crack prevention effect and to achieve maximum effects by the creasing processing.